About face groups

What is it?

Face groups, as in "groups of faces", where faces are triangles, are collections or lists of triangles. Typically, those triangles are contiguous, or directly attached to each other, but that is not a requirement; although when they are not, they usually form at least individual patches of contiguous triangles.

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How are face groups defined?

Typically, face groups are defined when importing a CAD model, where they derive from tessellating (converting to discrete triangles) individual surface sections described by B-rep, T-splines, or other parametric modeling methods. For each surface in a CAD model, a new collection of triangles is started, each such collection forms a new face group, but again, once defined there is no reason to also keep them individual.

In Netfabb in particular, which natively works on triangles only, face groups are defined in the Texture and color editor. Here, triangles intended to be in the same face group are given the same unique color. Then the color definition is used in a conversion to face groups.

Upon definition or derivation, a face group is given a unique ID internally.

Visible to the user, a face group gets a running number, a name, and a display color. The display color is only used for visualization in the Texture and color editor. It is also independent from the triangles' own actual color or the part's display color as a whole.

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How are face groups used?

CAD software tools, which natively work on parametric surfaces, tend to lack the tools to work well or at all with simple triangles. They usually need face groups to reconstruct the overall shape formed by the individual triangles and map a parametric surface to it before they can use their native parametric tools again.

In Netfabb, which natively works on triangles, not parametric surfaces, face groups help with various triangle-based tasks:

• During the cluster detection for creating supports, every field created by section or intersection of face groups with clusters is treated as a separate cluster.
• Also during cluster detection, face groups can be filtered for by name, making excluding them, or supporting them exclusively, much easier than having to construe detection parameters that happen to have the same filtering effect.
• When searching for part orientation, face groups are looked up to exclude areas from receiving support, thus affecting the ranking of orientation results.
• Manually slicing parts stores the face groups in the contours (as "tags") so that those can be treated separately even and especially when they form segments of the same closed contour.

Face groups are only stored within a FABBPROJECT, they are not maintained during export to any file format with the exception of CAD file formats where the part must have remained a CAD part since loading it into the project as Netfabb cannot convert mesh to parametric.

Face group IDs are permitted to be reused across parts, but they must be unique within one part. Triangles of the same face group ID effectively belong to the same face group.

• Duplicating parts with face groups, or cutting parts with face groups does not generate new unique IDs for the segments.
• Merging separate parts with face groups of the same ID also effectively merges the groups. An example of what merges separate parts is the Boolean unify.

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